1. Field of the Invention
The invention relates to wireless communication, and more particularly to a method and a system enabling roaming between different wireless networks in which, via a virtual GPRS support node, a plurality of data packets and control signals are delivered between low and high-tier wireless networks.
2. Description of the Related Art
A number of wireless network technologies have been proposed during the past few years. New radio access technologies and wireless network standards are also being developing. It is believed that multiple standards will coexist in the same environment for future wireless communication systems. Enabling seamless roaming between different networks is becoming more and more important in multiple standard environments. Different radio access networks have their own properties. High-tier systems such as General Packet Radio Service (GPRS) and Universal Mobile Telecommunication System (UMTS) provide high mobility with lower data transmission bandwidth. On the other hand, low-tier systems such as wireless local area network (Wireless LAN, WLAN) provides high data bandwidth but with less mobility.
FIG. 1 shows two different wireless networks 100 and 200 conventionally connected. Here, the wireless network 100 is a GPRS network used as an example of a high tier system. The wireless network 200 is a WLAN used as an example of a low tier system. As shown in FIG. 1, the GPRS network 100 comprises three base station systems (BSSs) 104a˜104c, three Serving GPRS Support Nodes (SGSNs) 106a˜106c, a Gateway GPRS Support Node (GGSN) 108a, a domain name server (DNS) 110 and a dynamic host configuration protocol (DHCP) Server 112. BSSs 104a˜104c can convert wireless signals to data. Cells of BSSs 104a˜104c are 102a˜102c. The cells 102a˜102c normally cover 500 meters to 30 km. SGSNs 106a˜106c relay data packets and regulate mobility management (GMM) and session management (SM) such as managing different routing areas (RAs) and mobile stations (MSs). GGSN 108a is an interface between the GPRS network 100 and an external network such as the Internet 300.
For embodied explanation of the WLAN, an example is given in the following. As shown in FIG. 1, the WLAN 200 comprises access points whose cells are 202a˜202i, two routers 206a and 206b, a gateway 208, a DNS 210 and a DHCP Server 212. The cells 202a˜202i normally cover 100 meters to 300 meters. Because the coverage of the cells in the WLAN is much lower than the cells in the GPRS network, the WLAN is normally installed in a “hot spot” area such as a building, station or airport. As well, one “hot spot” area installed the WLAN often comprises several access points.
For dual mode devices, there are several approaches to enable data communication in multiple networks. In the simplest approach, the two networks are used independently. FIG. 2 shows this example. The topology of the network in FIG. 2 is the same as in FIG. 1. Dotted line A in FIG. 2 shows the roaming route of the mobile device 40 supporting the GPRS standard and WLAN standard. The mobile device 40 attaches to the GPRS network 100 at point A1 and starts to access a remote host 42 through the Internet 300. Then, packets can be delivered between the mobile device 40 and the remote host 42. While the mobile device 40 detects the signal from the WLAN 200 becoming stronger than the previous one (GPRS network 100) such as at point A2, it stops the service from the GPRS network 100 and attaches to the new network (WLAN 200). In this approach, all of the current links and service will break since each network has its own network planning, routing, IP address and configurations.
In order to support unbreakable IP service during roaming, FIG. 3 shows another approach that introduces Mobile IP. The topology of the network in FIG. 3 is almost the same as in FIG. 2. The difference is that mobile IP devices 46a and 46b are added in FIG. 3. This approach provides unbreakable IP service. However, this approach requires installing mobile IP devices such as home agents and foreign agents 46a and 46b in both networks. Since the mobile device 40 requires return registration (to GPRS network 100), packet delay and loss will be also a problem during the period of handovers. Moreover, this approach suffers from triangle routing between the GPRS network 100 and the WLAN 200 if Mobile IP devices do not support route optimization (referring to dotted line C in FIG. 3).
Another approach is to let low tier system serve as a local radio access network under a high tier system. To connect a low tier base station to a high tier core network, an emulator is necessary. FIG. 4 shows connection of a WLAN base stations to a GPRS system. As shown in FIG. 4, the GPRS system comprises three base station systems (BSSs) 104a˜104c, three Serving GPRS Support Nodes (SGSNs) 106a˜105c, a Gateway GPRS Support Node (GGSN) 108a, a domain name server (DNS) 110, access points whose cells are 402a˜402i, a BSS emulator 404 and a SGSN emulator 406. Every WLAN base station (access point) can be regarded as a GPRS base station through the BSS emulator 404 or a SGSN through the SGSN emulator 406. The benefit of this approach is that no mobile IP is required. All packet routing and forwarding are processed by GPRS core network. Packet loss and delay are greatly reduced. However, this approach lacks flexibly since the two networks are tightly coupled. The operators of the two networks must be the same in order to exchange large amounts of information. Another disadvantage of this approach is that GGSN will be the single point access to the Internet. Packets through two networks must follow GGSN first, creating a bottleneck.